Previously, lower_jumps.cpp would break out of its loop after lowering
a jump instruction in just the then- or else-branch of a conditional,
and it would fail to lower a jump instruction occurring in the other
branch.
Without this patch, lower_jumps.cpp may require multiple passes in
order to lower all jumps. This results in sub-optimal output because
lower_jumps.cpp produces a brand new set of temporary variables each
time it is run, and the redundant temporary variables are not
guaranteed to be eliminated by later optimization passes.
Fixes unit test test_lower_returns_4.
e71b4ab8a6